This application claims priority from European Patent Application EP 011 08 180.9, filed on Mar. 30, 2001, and incorporated in full into this application by reference.
The invention relates to an onboard vehicle electrical system. More particularly, the invention relates to a vehicle electrical system that includes a low-voltage energy storage device, low-voltage consumers supplied by the energy storage device, and a starter generator.
Such a vehicle electrical system is known, for instance, from German Laid-Open Publication DE 197 52 661 A1. Vehicle electrical systems with a starter generator, coupled to an internal combustion engine, for automobiles generally have a 12/14V and a 36/42V voltage level. The introduction of the 36/42V level was in response to the increasing power requirements in the vehicle, particularly to cold-start the starter generator. Due to the correspondingly reduced current intensity, smaller winding cross sections can be selected for the machine configuration of the starter generator, and the losses in the inverter, which is fitted with semiconductor components and located between the starter generator and the DC voltage level, can be reduced.
For trucks or utility vehicles, the 24/28V voltage level has been introduced as a standard, also for reasons of compatibility between manufacturer-specific tractors and different types of trailers. For technical and economic reasons, it is unlikely that a 36/42V voltage level will be introduced in addition in the near future.
Two options are feasible to achieve a voltage that is higher than the relatively lower voltage of the vehicle electrical system and which is suitable to start the starter generator. The first, based on providing a low-voltage energy storage device, includes a step-up switching regulator fitted with semiconductor components and which raises the voltage supplying the starter generator. This can be done, e.g., using a start capacitor, e.g. as disclosed in the aforementioned DE 197 52 661 A1. As an alternative thereto, it would be possible, based on providing a higher-voltage energy storage device, to supply the low-voltage consumer components via a DC/DC converter fitted with semiconductor components.
One object of the present invention is to provide a vehicle electrical system that can be produced and operated at low cost and that permits the starting of the starter generator with current intensities that are not excessively high. It is a further object of the invention to provide a vehicle electrical system that is compatible with the conventional low voltage level, such as 24/28V for trucks, yet is nevertheless capable of supplying voltage to the low-voltage consumer components of the vehicle.
According to one formulation of the invention, these and other objects are achieved by a vehicle electrical system, in particular for a truck, that includes: (i) a first low-voltage vehicle electrical system energy storage device; (ii) low-voltage vehicle electrical system consumers supplied with voltage by the first energy storage device; (iii) a starter generator, which, in starter operation, is supplied with voltage from the first energy storage device and, in generator operation, re-supplies the first energy storage device with voltage; and (iv) an additional low-voltage energy storage device, which, in starter operation of the starter generator, is connected in series with the first energy storage device for a higher voltage for the starter generator and, in generator operation of the starter generator, is connected in parallel with the first energy storage device for a lower voltage.
German Laid-Open Publication DE 43 02 809 A1 (and its related publications FR 2 687 511 and U.S. Pat. No. 5,455,463) disclose a device to regulate or control the onboard system voltage of a vehicle during operation with a high current consumer in the form of a starter for an internal combustion engine using a main vehicle battery and an auxiliary battery. However, in contrast to the present invention, that device, in order to prevent side effects on other current consumers when the high current consumer is connected, provides that the auxiliary battery be connected in series, preferably with a reduced voltage, with the main battery, and that the high current consumer be supplied from the main battery and the remaining consumers from the series connection of the batteries.
The present invention provides a demand-adjusted series or parallel connection of the two energy storage devices. Starting from a compatible,low voltage of a basic voltage level (e.g., 24/28V) that corresponds to the requirements of the vehicle electrical system consumers, an advantageous, noticeably higher starting voltage for the starter generator (e.g., 48/56 V) can nevertheless be obtained to start, particularly cold-start, the starter generator, which is a strong consumer and which is typically coupled to an internal combustion engine. This is achieved merely by actuating switches, i.e., without resort to step-up switching regulators or high-voltage energy storage devices that cannot otherwise be used or can be used only with DC/DC chopper converters. At the same speed and torque, this results in a lower current per winding for the starter generator, a lower output current of the inverter, or a reduced load current for the energy storage device(s). The switching components required for switching between the series and parallel connections can be produced with little complexity and are simple to control.